The reactivity of gas phase radicals generated during the MOCVD of GaAs, GaN, ZnS, ZnSe, and CdTe was studied using density functional theory. Calculations were performed with B3LYP functionals and different basis sets. It was found that the production of CH3 through surface reactions is an intrinsic part of the growth process when using Ga(CH3)(3), Zn(CH3)(2), or Cd(CH3)(2) as gas phase precursors. After being generated, CH3 can react with the organometallic molecules or with H,, if it is used as carrier gas, to give CH4 and H. Atomic hydrogen is much more reactive than CH3 and can easily decompose Ga(CH3)(3) and Cd(CH3)(2) to GaCH3 and CdCH3, which leads to an increase of the film growth rate. When reacting with H,Se, H can start a radical chain mechanism that can eventually determine the formation of large ZnSe gas phase adducts. Finally, it was found that H can react with NH3 to give NH2, which can then react with Ga(CH3)(3) to give Ga(CH3)(2)NH2 and successively form GaN gas phase adducts. (C) 2004 Elsevier B.V. All rights reserved.